A Lyot filter is an optical filter that passes a relatively narrow band of wavelengths by using birefringent materials, such as plates made from quartz. A conventional Lyot filter is often formed by using one or more of these birefringent plates oriented at Brewster's angle. Each plate corresponds to a stage of the Lyot filter. For example, a typical three-stage Lyot filter has three birefringent plates. When more than one birefringent plate is used in a Lyot filter, each plate added is twice the size of the previous plate. For a three-stage Lyot filter, therefore, the thicknesses of the three plates are d, 2 d and 4 d. Thus, in order to implement a multi-stage Lyot filter, the plates have to be manufactured precisely to provide the desired thicknesses relative to each other. In addition, Lyot filters generally include parallel linear polarizers before and after each stage.
In order to tune a conventional multi-stageLyot filter to a particular wavelength, each of the separate plates has to be rotated. In addition, an uncoatedLyot filter at Brewster's angle has limited hold-off when tuning away from the gain peak is attempted. Other types of Lyot filters have been implemented that use angle-tuning of a diffraction grating or a volume Bragg grating mirror. However, these types of filters also require some type of rotation for tuning. Some multi-stageLyot filters have been implemented using electro-optic crystals. However, in order to tune these filters, the voltage applied to each of the electro-optic crystals will likely have to be individually adjusted. Lyot filters with liquid crystals or piezo-tunable etalons are also available. However, these types of filters are not useful for intracavity tuning of Q-switched lasers because liquid crystals have low damage thresholds and high-finesse etalons have internal intensities that are significantly higher than that of the resonator cavity.